Conventionally, nematic crystal has been typically used for liquid crystal display apparatuses. Such liquid crystal display apparatuses include Twisted Nematic (TN) type liquid crystal display apparatuses and Super-Twisted Nematic (STN) type liquid crystal display apparatuses which have been improved from the TN type liquid crystal display apparatuses.
For the TN type liquid crystal display apparatus, since threshold characteristics of light transmittance dependent on voltage show a response which is not quick enough, a drive margin becomes narrower as drive system is made highly multiplex and enough contrast cannot be obtained. The STN type liquid crystal display apparatus is able to solve this defect by orientating liquid crystal molecules with a large twist angle. However, the STN type liquid crystal display apparatus deteriorates in contrast and response speed when applied for a large capacity display.
To solve the defect, N. A. Clark and S. T. Lagerwall suggests in "Applied Physics Letters" 36, (1980) p. 899-901, a liquid crystal display apparatus using chiral smectic C liquid crystal, i.e., ferroelectric liquid crystal. The liquid crystal display apparatus uses turning force to conform the polarity of spontaneous polarization of the ferroelectric liquid crystal and the polarity of an electric field, instead of driving nematic liquid crystal using dielectric anisotropy of liquid crystal molecules.
As shown in FIGS. 20(a) and 20(b), the ferroelectric liquid crystal in a bulk state has, over a helical pitch, a molecule arrangement where the longer axes of liquid crystal molecules 101 are inclined by a definite angle .theta. to the normal z of a layer surface 102 forming a boundary with a smectic layer, and the inclination direction revolves around the normal z from layer to layer by a definite angle .PHI.. Each liquid crystal molecule 101 has spontaneous polarization 103 perpendicular to both the longer axis and the normal z.
The ferroelectric liquid crystal un winds the helical structure when injected into a cell 104 where two electrode substrates provided with polarizing plates are combined with a smaller gap therebetween than the helical pitch, as shown in FIG. 21(a). This creates coexistence of two stable states: a domain where the liquid crystal molecules 101 are stable when tilted to the normal z by the tilt angle +.theta. and a domain where the liquid crystal molecules 101 are stable when tilted to the normal z by the tilt angle -.theta. in the opposite direction.
Then, an electric field is applied to the ferroelectric liquid crystal in a state where the longer axes of the liquid crystal molecules 101 agree with the polarizing axis 105 of one of the polarizing plates. Thus, the longer axis and the direction of the spontaneous polarization 103 of the liquid crystal molecule 101 agree as shown in FIG. 21(b). Here, if a negative electric field is applied, since double-refraction effect does not occur, incident light is not double-refracted. From this state, as shown in FIG. 21(c), a switching operation of switching the orientation of the liquid crystal molecules 101 from a definite state to another state is carried out by switching the polarity of the applied electric field.
Incident light is double-refracted in the ferroelectric liquid crystal in the cell 104 for these reasons. Therefore, light transmission is controlled by holding the cell 104 with two polarizers so as to create a dark state (FIG. 21(b)) and a bright state (FIG. 21(c)).
In addition, if the electric field is removed, the orientation of the liquid crystal molecules 101 is maintained by orientation anchoring force of the boundary plane so as to be in a state when the electric field is applied. This is a feature of memory effect of ferroelectric liquid crystal. Moreover, since the electric field directly affects the spontaneous polarization 103, the switching can respond quickly, within less than one thousandth of the time of nematic liquid crystal display apparatuses. A quick display becomes possible in this manner.
Ferroelectric liquid crystal has useful characteristics, such as a wide viewing angle as well as the above-mentioned bistability, memory effect and quick response. Therefore, in recent years, a lot of efforts have been made to apply ferroelectric liquid crystal to liquid crystal display apparatuses of high definition and a large capacity.
However, the aforementioned ferroelectric liquid crystal has a lot of problems. First of all, since chiral smectic C phase liquid crystal showing ferroelectricity has molecular environment of low symmetry and high crystallinity, compared to ordinary nematic liquid crystal, it is difficult to orient molecules uniformly. Therefore, it is difficult to realize uniform display.
Initial studies about ferroelectric liquid crystal assumed that ferroelectric liquid crystal had a layer structure called a bookshelf type where the smectic C phases are arranged perpendicularly to the substrate. However, it has been found out in a cell made according to a uniaxial orientation method of conventional rubbing and the like that switching phenomena and optical characteristics greatly differ from those expected and that a switching operation totally different from a suggested molecular orientational model is carried out.
It is found out with a small angle scattering method using X ray that this is mainly caused by a mid-bent structure called chevron of each smectic layer 107 formed between substrates 106 as shown in FIG. 22. The structure has C1 and C2 states according to relationship between a pretilt angle at a boundary plane of the liquid crystal and the substrate 106 and the twist direction of the smectic layer 107 as shown in FIGS. 23(b) and 23(c). There is another classification for the structure: uniform orientation where liquid crystal molecules are uniformly oriented between the opposite substrates 106, and a twisted orientation where the liquid crystal molecules are twisted and oriented.
Since the molecules are almost uniaxially oriented in the uniform orientation, the uniform orientation shows extinction positions with the two polarizing plates whose polarization axes perpendicularly cross. Nevertheless, the twisted orientation shows no extinction positions, since the twisted molecules generate rotatory polarization. Consequently, ferroelectric liquid crystal exhibits four orientational states: C1U (C1-Uniform), C2U (C2-Uniform), C1T (C1-Twisted) and C2T (C2-Twisted) (see "LIQUID CRYSTALS" 1993, Vol. 15, No. 5, p. 669-p. 687).
Among these orientations, the C2U orientation is appropriate for display apparatuses, because, for example, the C2U orientation produces high contrast (see "Ferroelectrics" 1993, Vol. 149, pp. 183-192 and "JAPANESE JOURNAL OF APPLIED PHYSICS" VOL. 33, No. 4A, APRIL, 1994, pp. 1972-1976).
In the chevron structure, so-called zig zag defects are observed where the smectic layers 107 incline in different directions as shown in FIG. 23(a). There are two kinds of zig zag defects: a roundly-curving hairpin defect occurring in the rubbing direction and a sharply-turning lightening defect occurring in the opposite direction from the rubbing direction.
It is needless to say that these defects pose a problem in obtaining uniform display characteristics, and optical characteristics vary greatly depending upon orientational states sandwiching the defects. Therefore, in order to prevent the zig zag defects from occurring, it is important to control bending direction of the smectic layer to be uniform (see "LIQUID CRYSTAL").
A state where the uniform orientation and the twisted orientation coexist in mixture results in low contract and flickering in display, and therefore is not preferable for a display apparatus. It is assumed that the uniform and twisted orientations are determined by spontaneous twisting force of the liquid crystal, molecular interaction between the liquid crystal and an orientating film, polarity energy and the like. Therefore, the effects of the coexistence of the two orientations are controlled by adjusting combination of materials for the liquid crystal and the orientating film.
Meanwhile, it is assumed that stability of the C1 and C2 states are determined by geometric effects, such as a relation between a layer inclination angle and a tilt angle of the liquid crystal and a pretilt angle .theta..sub.p of the orientating film (FIG. 23(b)). Therefore, conventionally, it is suggested to control the relation of the angles (see "Ferroelectrics" 1991, Vol. 114, pp. 3-26). However, since the control is incomplete, the control is not a practical and effective solution.
This is presumably because the inclination direction of the smectic layer 107 reverses suddenly not only because of the relation between the tilt angle and the layer inclination angle of the liquid crystal and the pretilt angle, but also because of a foreign substance like a spacer disposed between the substrates as disclosed in Japanese Laid-Open Patent Application No. 1-179915/1989 (Tokukaihei 1-179915). In this case, it is assumed that the spacer serves as a core causing the zig zag defects.
Since chiral smectic C liquid crystal has high crystallinity and low fluidity, a mechanical shock breaks smectic layer structure thereof. Therefore chiral smectic C liquid crystal does not return to the original orientational state unless heated up to a temperature higher than a temperature where chiral smectic C liquid crystal exhibits a nematic phase. In order to solve this problem, a shock weakening method needs to be adopted.
For example, Japanese Publication for Examined Patent Application No. 2-17007/1990 (Tokukouhei 2-17007) suggests provision of a cylindrical or a strap-shaped protrusion in a cell, using photosensitive polyimide and the like. Besides, Japanese Laid-Open Patent Application No. 63-116126/1988 (Tokukaishou 63-116126) suggests solid bonding of opposite substrates with adhesive spacers. The protrusion and the spacers suggested in these methods prevent the substrates from bending with a shock, and therefore the shock does not reach the crystal liquid. However, these structures may result in a small interval between the spacers and thereby damage injectability of the liquid crystal. Therefore the liquid crystal may not injected into some pixels.